1. Field of the Invention
The present invention relates to boundary acoustic wave devices using boundary acoustic waves propagating along an interface between a piezoelectric substrate and a dielectric layer, and more particularly, to a boundary acoustic wave device having a three-media structure including a second medium layer made of a first dielectric material disposed on a first medium layer made of a second dielectric material.
2. Description of the Related Art
Boundary acoustic wave devices have recently received significant attention as an alternative to surface acoustic wave devices. Boundary acoustic wave devices do not require hollow packages. Accordingly, the size of resonators and filters can be significantly reduced by using boundary acoustic wave devices.
WO 98/52279 discloses a boundary acoustic wave device having a three-layer structure including a first medium layer and a second medium layer disposed on a piezoelectric substrate. As shown in FIG. 8, an interdigital electrode (not shown) is arranged on a piezoelectric substrate 1001, and a polycrystalline silicon oxide film is disposed as the first medium layer 1002 on the piezoelectric substrate 1001. A polycrystalline silicon film is disposed as the second medium layer 1003 on the first medium layer 1002. The IDT electrode is arranged along the interface between the piezoelectric substrate and the polycrystalline silicon oxide film.
By forming the polycrystalline silicon film, as shown in FIG. 8, the boundary acoustic wave energy excited by the IDT electrode is confined in the first medium layer 1002 made of polycrystalline silicon oxide.
Also, even if the polycrystalline silicon film is degraded or deteriorated, the electrical properties are not substantially impaired. In addition, since the polycrystalline silicon oxide film and the polycrystalline silicon film protect the IDT electrode, the reliability is improved. Furthermore, the use of a three-media structure can produce a high-frequency device.
However, in the boundary acoustic wave device having the three-media structure disclosed in WO 98/52279, higher-order mode spurious responses often occur. The magnitude of the higher-order mode spurious responses varies depending on the thickness of the silicon oxide film.
The magnitude of spurious responses can be reduced by reducing the thickness of the silicon oxide film.
Unfortunately, if the thickness of the silicon oxide film is reduced, the temperature coefficient of frequency (TCF) of the boundary acoustic wave device becomes negative, and its absolute value is increased. This means that the suppression of higher-order mode spurious responses and the improvement in TCF have a trade-off relationship.